Method for manufacturing light-selective prism

ABSTRACT

The present invention provides a technique for improving the optical characteristics of a light-selective prism. By cutting a block along a plane perpendicular to a first direction, a plurality of first small blocks are obtained. A first selective film is formed on the cut face of the first small block, and the plurality of first small blocks are stuck together so as to obtain a first processed block in which the first selective film is located at the interface of adjacent two first small blocks. Next, by cutting the first processed block along a plane perpendicular to a second direction that is substantially perpendicular to the first direction, a plurality of second small blocks are obtained. A second selective film is formed on the cut face of the second small block, and the plurality of second small blocks are stuck together to obtain a second processed block in which the second selective film is located at the interface of adjacent two second small blocks. Light-selective prisms in which two kinds of selective films are formed on an almost X shape interface is obtained from the second processed block.

TECHNICAL FIELD

The present invention relates to a light-selective prism for use in aprojector or the like.

BACKGROUND ART

In projectors, light emitted from an illuminating optical system ismodulated by means of a liquid crystal light valve in response to imageinformation (image signal), and the modulated light is projected onto ascreen.

In projector for projecting color images, a colored light separatingoptical system is provided for separating light emitted from anilluminating optical system into three colored lights, and a coloredlight combining optical system for combining three modulated lightsemitted from three liquid crystal light valves. As a colored lightcombining optical system, a light-selective prism (cross dichroic prism)is used for example, having two types of selective films formed at analmost “X” shape interface of four rectangular prisms.

Selective prisms are conventionally manufactured by independentlypreparing four rectangular prisms and then sticking them together. Amethod for manufacturing a selective prism of this kind is described,for example, in JAPANESE PATENT LAID-OPEN GAZETTE No. H11-352440disclosed by the present applicant.

However, where four rectangular prisms are prepared independently in theabove manner, sometimes the desired optical characteristics of thelight-selective prism are not obtained. One cause of this is that thereis variation in the refractive index of each rectangular prism. Thisvariation of refractive index can occur, for example, by usinglight-transmissive parts of different lots to form each rectangularprism.

DISCLOSURE OF THE INVENTION

The object of the present invention is thus to solve the drawbacks ofthe prior art discussed above and to provide a technique for improvingoptical characteristics of a light-selective prism.

At least part of the above and the other related issues are solvedthrough the method for manufacturing a light-selective prism in thepresent invention. The light-selective prism has a substantially regulartetragonal columnar outer shape and includes two kinds of selectivefilms, formed on an almost X shape interface, each selective filmselecting colored light having wavelengths of a predetermined range. Themanufacturing method comprises the steps of: (a) preparing a blockformed of a light transmissive member; (b) cutting the block along atleast one plane perpendicular to a first direction so as to obtain aplurality of first small blocks whose dimension in the first directionis substantially equal to a predetermined dimension; (c) forming a firstselective film on a cut face of at least one of the plurality of thefirst small blocks; (d) sticking the plurality of the first small blockstogether so as to obtain a first processed block in which the firstselective film is situated at an interface of adjacent two first smallblocks; (e) cutting the first processed block along at least one planeperpendicular to a second direction that is substantially perpendicularto the first direction so as to obtain a plurality of second smallblocks whose dimension in the second direction is substantially equal tothe predetermined dimension; (f) forming a second selective film on acut face of at least one of the plurality of the second small blocks;(g) sticking the plurality of the second small blocks together so as toobtain a second processed block in which the second selective film issituated at an interface of adjacent two second small blocks; and (h)obtaining at least one light-selective prism from the second processedblock.

With the method of the present invention, the light-selective prism ismanufactured from the single block. Thus, variation in refractive indexof light-transmissive parts constituting the light-selective prism thatoccurs due to differences among lots can be reduced, and as a result ofthis, it is possible to improve optical characteristics of thelight-selective prism.

In the above method, it is preferable that the step (b) includespolishing the cut faces of the first small blocks; and that the step (e)includes polishing the cut faces of the second small blocks.

By so doing, the film formation face of a first small block on which afirst selective film is formed, and the film formation face of a secondsmall block on which a second selective film is formed, can beplanarized, so the degree of adhesion of the first and second selectivefilms to the first and second small blocks can be improved,respectively.

Here, the step (h) may include cutting the second processed block so asto obtain a plurality of the light-selective prisms.

By so doing, the plurality of the light-selective prisms can be obtainedfrom the single block.

In the above method, the step (d) may include sticking the plurality ofthe first small blocks together such that the outer shape of the blockis reproduced; and the step (g) may include sticking the plurality ofthe second small blocks together such that the outer shape of the blockis reproduced.

Here, it is preferable that the step (d) includes sticking the pluralityof the first small blocks together such that each portion of the lighttransmissive member constituting each first small block is placed in thesame location within the block; and the step (g) includes sticking theplurality of the second small blocks together such that each portion ofthe light transmissive member constituting each second small block isplaced in the same location within the block.

Even within a single block, there are instances in which there isvariation in the refractive index depending on spatial position. Thus,by proceeding in the manner described above, spatial variation inrefractive index of light-transmissive parts constituting alight-selective prism can be reduced, and it becomes possible to improvethe optical characteristics of the light-selective prism.

Alternatively, the step (d) may include sticking the plurality of thefirst small blocks together in a state such that adjacent two firstsmall blocks are dislocated in a direction substantially perpendicularto the first and the second directions; and the step (g) may includesticking the plurality of the second small blocks together such that anouter shape of the first processed block is reproduced.

In this arrangement, the plurality of second small blocks can be stucktogether by utilizing the dislocation formed between two adjacent firstsmall blocks. Therefore, when sticking the plurality of second smallblocks together, the first selective film portions divided due tocutting out of the plurality of second small blocks can be easilyarranged within the same plane.

Here, it is preferable that the step (d) includes sticking the pluralityof the first small blocks together such that each portion of the lighttransmissive member constituting each first small block is placed insubstantially the same location within the block; and the step (g)includes sticking the plurality of the second small blocks together suchthat each portion of the light transmissive member constituting eachsecond small block is placed in the same location within the firstprocessed block.

In the above method, it is preferable that the first selective film is ablue light reflecting film for selectively reflecting blue light; andthat the second selective film is a red light reflecting film forselectively reflecting red light.

Within the light-selective prism, the first selective film is formed ina divided state, but the second selective film is formed in a continuousstate. The sensitivity of the human eye is higher to red light than toblue light. Therefore, by setting the first selective film and thesecond selective film to a blue light reflecting film and a red lightreflecting film respectively, segmentation of the first selective filmdoes not stand out compared to the case of the reverse setting.

In the above method, it is preferable that the block has a substantiallyrectangular parallelopiped shape, and that the at least one planeperpendicular to the first direction and the at least one planeperpendicular to the second direction are set to planes inclined byabout 45 degrees with respect to each side of one pair of opposing facesof the block.

By so doing, the light-transmissive member that forms a block can beutilized without waste, and at least one light-selective prism can beobtained.

In the above method, it is preferable that the light transmissive memberis a member having a thermal conductivity of at least about 5.0 W/(m·K).

By so doing, the temperature rise of the light-selective prism per secan be reduced. Further, when an optical component of relatively largeheat generation such as a polarizing plate or retardation plate isattached to the light-selective prism, temperature rise of these opticalcomponents can be reduced as well.

In the above method, it is preferable that the light transmissive memberis a uniaxial crystal member, and that the first and second directionsare set to directions substantially perpendicular to an optic axis ofthe uniaxial crystal.

Here, the uniaxial crystal member may be a monocrystalline sapphiremember or a rock crystal member.

Uniaxial monocrystalline members can be used as light-transmissivemembers with relatively high thermal conductivity. However, when linearpolarized light enters a uniaxial monocrystalline, the state ofpolarization thereof is changed in some cases. If the relationship ofthe first and second directions and the optic axis of the monocrystal isset as mentioned above, the state of polarization of linear polarizedlight will not be changed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing a projector including a crossdichroic prism manufactured by implementing the present invention;

FIG. 2 is an explanatory diagram showing a relevant portion of theprojector 100 of FIG. 1;

FIG. 3 is a flow chart showing a method for manufacturing a crossdichroic prism 500;

FIG. 4 is an explanatory diagram showing a prepared block 600;

FIG. 5 is an explanatory diagram showing the aspect of cutting the block600;

FIG. 6 is an explanatory diagram showing the No. 4 first small block 614enlarged;

FIG. 7 is an explanatory diagram showing a first processed block 600A;

FIG. 8 is an explanatory diagram showing the aspect of cutting the firstprocessed block 600A;

FIG. 9 is an explanatory diagram showing the No. 4 second small block624 enlarged;

FIG. 10 is an explanatory diagram showing a second processed block 600B;

FIG. 11 is an explanatory diagram showing the aspect of cutting thesecond processed block 600B;

FIG. 12 is an explanatory diagram showing one manufactured crossdichroic prism 500 enlarged;

FIG. 13 is an explanatory diagram showing a cross dichroic prism 510manufactured by using a monocrystalline sapphire member;

FIG. 14 is an explanatory diagram showing a block 610 formed by amonocrystalline sapphire member prepared to obtain the cross dichroicprism 510 of FIG. 13;

FIG. 15 is an explanatory diagram showing a first example of a crossdichroic prism in which the segmented two first selective films are notarranged in the same plane;

FIG. 16 is an explanatory diagram showing a second example of a crossdichroic prism in which the segmented two first selective films are notarranged in the same plane;

FIG. 17 is an explanatory diagram showing a first processed block 602Aobtained in Step S105 of the third embodiment, and corresponds to FIG.7;

FIG. 18 is an explanatory diagram showing the aspect of cutting thefirst processed block 602A, and corresponds to FIG. 8;

FIG. 19 is an explanatory diagram representatively depicting aspect ofthe polishing process; and

FIG. 20 is an explanatory diagram showing a second processed block 602Bobtained in Step S109 of the third embodiment, and corresponds to FIG.10.

BEST MODE FOR CARRYING OUT THE INVENTION A. First Embodiment

A-1. General Structure of Projector:

One mode of carrying out the present invention is discussed below as apreferred embodiment. FIG. 1 is an explanatory diagram showing aprojector including a cross dichroic prism manufactured by implementingthe present invention. The projector 1000 comprises: an illuminatingoptical system 100, a colored light separating optical system 200, arelay optical system 220, three liquid crystal light valves 300R, 300G,300B, a cross dichroic prism 500, and a projecting optical system 540.

The illuminating optical system 100 includes a polarized lightgenerating optical system 160, converts light emitted from a lightsource device 120 into a single kind of linearly polarized light havingthe same polarization direction and emits the converted light. The lightemitted from the illuminating optical system 100 is separated by thecolored light separating optical system 200 into three colored lights,red (R), green (G) and blue (B). The separated colored lights aremodulated by liquid crystal light valves 300R, 300G, 300B in response tothe image information (image signal). The three modulated colored lightsare combined by cross dichroic prism 500, and the composite light isprojected onto a screen SC by the projecting optical system 540. Thisarrangement allows a color image to be displayed on the screen SC.Regarding the structure and function of parts of the projector shown inFIG. 1, as there is a detailed description, for example, in JAPANESEPATENT LAID-OPEN GAZETTE No. H10-325954 disclosed by the presentapplicant, a detailed description is omitted in this specification.

FIG. 2 is an explanatory diagram showing a relevant portion of theprojector 100 of FIG. 1. In FIG. 2, three liquid crystal light valves300R, 300G, 300B and the cross dichroic prism 500 of FIG. 1 are shown.

Colored lights R, G, B enter the first through third liquid crystallight valves 300R, 300G, 300B, respectively. Modulated light (linearlypolarized light) of colored light R emitted from the first liquidcrystal light valve 300R is reflected by the red light reflecting filmRR of the cross dichroic prism 500, and modulated light (linearlypolarized light) of colored light B emitted from the third liquidcrystal light valve 300B is reflected by the blue light reflecting filmRB. On the other hand, modulated light (linearly polarized light) ofcolored light G emitted from the second liquid crystal light valve 300Gpasses through the two reflecting films RB, RR of the cross dichroicprism 500. In this way, the three modulated lights are combined, and acolor image is displayed on screen SC by the projecting optical system540. In FIG. 2, for convenience of illustration, the positions at whichred light and blue light are reflected are portrayed at locationsshifted away from the two reflecting films RB, RR.

The first liquid crystal light valve 300R comprises a liquid crystalpanel 301R, and two polarizing plates 302Ri, 302Ro, provided on thelight incident side and light exiting side thereof. The first polarizingplate 302Ri provided on the light incident side is attached to theliquid crystal panel 301R. On the other hand, the second polarizingplate 302Ro provided on the light exiting side is attached on a lighttransmissive substrate 308 at a location away from the liquid crystalpanel 301R.

Colored light R incident on the first liquid crystal light valve 300R isemitted from the illuminating optical system 100 (FIG. 1) which includesa polarized light generating optical system 160, and is thereforelinearly polarized light. The polarization axis of the first polarizingplate 302Ri provided on the light incident side of liquid crystal lightvalve 300R is set so as to be the same as the polarization direction ofincident linearly polarized light. Thus, colored light R incident on thefirst polarizing plate 302Ri mostly passes through the first polarizingplate 302Ri. Polarized light emitted from first polarizing plate 302Riis modulated by liquid crystal panel 301R. The second polarizing plate302Ro allows transmission of only that light component having apolarization direction the same as the polarization axis thereof, fromthe components of light modulated by liquid crystal panel 301R.Modulated light (linearly polarized light) emitted from the secondpolarizing plate 302Ro passes through light transmissive substrate 308and enters cross dichroic prism 500.

In the above-described manner, the first polarizing plate 302Ritransmits substantially all incident linearly polarized light, whereasthe second polarizing plate 302Ro blocks some of the incident modulatedlight. Thus, the second polarizing plate 302Ro generates greater heatthan does the first polarizing plate 302Ri. In FIG. 2, in order toreduce the temperature rise of the second polarizing plate 302Ro whichexperiences relatively high heat generation, the second polarizing plate302Ro only is provided on a separately provided light transmissivesubstrate 308 that has a relatively high thermal conductivity. Thesecond and third liquid crystal light valves 300G, 300B are arrangedsimilarly.

Incidentally, cross dichroic prism 500 combines the three modulatedcolored lights (linearly polarized light) emitted from the liquidcrystal light valves, and the optical characteristics of cross dichroicprism 500 are highly dependent on its manufacturing method. In thisembodiment, by manipulating the manufacturing method of the crossdichroic prism, its optical characteristics are improved.

A-2. Manufacturing Method for Cross Dichroic Prism:

FIG. 3 is a flow chart showing a method for manufacturing a crossdichroic prism 500. In this embodiment, a plurality of cross dichroicprisms 500 are manufactured simultaneously. One of these is then used inthe projector 1000 shown in FIG. 1.

In Step S101, a block formed of a light transmissive member is prepared.FIG. 4 is an explanatory diagram showing a prepared block 600. The block(hereinafter also referred to as “original block”) 600 has an outershape of a substantially rectangular parallelopiped form, and is formedof glass. Specifically, the block 600 is produced by the press formingof melting glass using a mold.

In Step S102, by cutting the block 600, a plurality of first smallblocks are obtained. FIG. 5 is an explanatory diagram showing the aspectof cutting the block 600. In this embodiment, as shown in the drawing,the block 600 is cut along seven planes indicated by the dashed lines,so as to obtain eight first small blocks 611-618. These seven planes areplanes perpendicular to a first direction D1, in other words, planeshaving the first direction D1 as the normal, and planes inclined byabout 45 degrees with respect to each side of an opposing pair of facesS1, S2 of the block 600. The seven planes are also set such thatdistances between two adjacent planes are substantially equal. Thus, thedimension L1 in the first direction D1 is substantially the same in eachof the first small blocks 611-618. No. 1 and No. 8 first small blocks611, 618 have outer shapes of substantially triangular column form, andNo. 2 through No. 7 first small blocks 612-617 have outer shapes ofsubstantially tetragonal column form.

In Step S103, the cut faces of first small blocks 611-618 are polished.Specifically, regarding the two first small blocks 611, 618 located atthe two ends, the one cut face of each is polished, and regarding theother six first small blocks 612-617, the two cut faces of each arepolished. If the cut faces of the first small blocks are relativelyflat, the polishing process in Step S103 can be omitted.

In Step S104, first selective films are selectively formed for the eightfirst small blocks 611-18. Here, a blue light reflecting film thatselectively reflects blue light is formed as the first selective film .In this embodiment, a blue light reflecting film is formed on onepolished cut face of each of seven blocks, namely, the No. 2 through No.8 first small blocks 612-618. More specifically, a blue light selectivefilm is formed on one of the two cut faces at which two adjacent firstsmall blocks were in contact. FIG. 6 is an explanatory diagram showingthe No. 4 first small block 614 enlarged. As shown in the drawing, theNo. 4 first small block 614 has two cut faces Sa, Sb, and the blue lightreflecting film RB is formed on the one face Sa that is closer to theNo. 1 first small block 611 (FIG. 5). The same is true for the other sixfirst small blocks 612-613, 615-618. However, the No. 8 first smallblock 618 has only one cut face, and the blue light reflecting film RBis formed on this face. By forming a first selective film on a polishedcut face, it is possible to improve adhesion of the first selective filmto the first small block.

In Step S105, by sticking the eight first small blocks 611-618 togetherwith adhesive, a first processed block is produced. FIG. 7 is anexplanatory diagram showing a first processed block 600A. As shown inthe drawing, the first processed block 600A has an outer shape ofsubstantially rectangular parallelopiped form, and the outer shape ofthe original block 600 of FIG. 4 is reproduced. The eight first smallblocks 611-618 are stuck together in a condition such that the bluelight reflecting film RB is arranged at the interface of two adjacentfirst small blocks. The eight first small blocks 611-618 are also stucktogether in such a way that each portion of the light transmissivemember constituting each small block is placed in the same locationwithin the original block 600.

At this time, it is preferable that the six faces of the first processedblock 600A are polished. This is done to remove film material depositedon portions other than the face on which film is supposed to be formedduring forming of the blue light reflecting film RB in Step S104, and toremove adhesive deposited on portions other than the face that issupposed to be bonded during sticking of the first small blocks togetherin Step S105. By so doing, the six faces of the first processed block600A can be designated as reference faces for the cutting process inStep S106.

In Step S106, in the same manner as Step S102, by cutting the firstprocessed block 600A, a plurality of second small blocks are obtained.FIG. 8 is an explanatory diagram showing the aspect of cutting the firstprocessed block 600A. In this embodiment, as shown in the drawing, thefirst processed block 600A is cut along seven planes indicated by thedashed lines so as to obtain eight second small blocks 621-628. Theseseven planes are planes perpendicular to a second direction D2, in otherwords, planes having the second direction D2 as the normal, and planesinclined by about 45 degrees with respect to each side of an opposingpair of faces S1A, S2A of the first processed block 600A. The opposingpair of faces S1A, S2A of the first processed block 600A is the same asthe opposing pair of faces S1, S2 of the original block 600 shown inFIG. 5. The second direction D2 is a direction substantiallyperpendicular to the first direction D1, in other words, substantiallyparallel to the faces on which the blue light reflecting films RB wereformed. Also, the seven planes are set such that distances between twoadjacent planes are substantially equal. Thus, the dimension L2 in thesecond direction D2 is substantially the same in each second small block621-628. Dimension L2 of the second small blocks is set to the samevalue as dimension L1 of the first small blocks (FIG. 5).

In Step S107, in the same manner as in Step S103, the cut faces of thesecond small blocks 621-628 are polished. If the cut faces of the secondsmall blocks are relatively flat, the polishing process in Step S107 canbe omitted.

In Step S108, second selective films are selectively formed for theeight second small blocks 621-628. Here, as the second selective film, ared light reflecting film is formed that selectively reflects red light.In this embodiment, a red light reflecting film is formed on onepolished cut face of each of seven blocks, namely, the No. 2 through No.8 second small blocks 622-628. More specifically, a red light selectivefilm is formed on one of the two cut faces at which two adjacent secondsmall blocks were contacting. FIG. 9 is an explanatory diagram showingthe No. 4 second small block 624 enlarged. As shown in the drawing, theNo. 4 second small block 624 has two cut faces Sc, Sd, and the red lightreflecting film RR is formed on the one face Sc that is closer to theNo. 1 second small block 621 (FIG. 8). The same is true of the other sixsecond small blocks 622-623, 625-628. By forming a second selective filmon a polished cut face, it is possible to improve adhesion of the secondselective film to the second small block.

In Step S109, in the same manner as in Step S105, by sticking the eightsecond small blocks 621-628 together with adhesive, a second processedblock is produced. FIG. 10 is an explanatory diagram showing a secondprocessed block 600B. As shown in the drawing, the second processedblock 600B has the outer shape of a substantially rectangularparallelopiped form, and the outer shape of the original block 600 ofFIG. 4 is reproduced. The eight second small blocks 621-628 are stucktogether in a condition such that the red light reflecting film RR isarranged at the interface of two adjacent second small blocks. The eightsecond small blocks 621-628 are also stuck together in such a way thateach portion of the light transmissive member constituting each smallblock is placed in the same location within the original block 600.

At this time, as described in Step S105, it is preferable that the sixfaces of the second processed block 600B are polished.

In Step S110, by cutting the second processed block 600B, a plurality ofcross dichroic prisms are obtained. FIG. 11 is an explanatory diagramshowing the aspect of cutting the second processed block 600B. As shownin the drawing, by cutting the second processed block 600B along aplurality of planes indicated by dashed lines, 45 cross dichroic prismscan be obtained having a substantially regular tetragonal columnarshape. The three kinds of planes indicated by dashed lines correspond toplanes parallel to the three pairs of opposing faces of the secondprocessed block 600B.

As will be understood from FIG. 11, dimension L1 in direction D1 of eachfirst small block 611-618 shown in FIG. 5 and dimension L2 in directionD2 of each second small block 621-628 shown in FIG. 8 are (2^(1/2)/2)times the dimension L0 of one side of a face at which the almost “X”shaped interface of one cross dichroic prism 500 appears.

In the manner described above, a plurality of cross dichroic prisms 500can be manufactured simultaneously. The six faces of each cut crossdichroic prism 500 are to be polished.

Incidentally, in this embodiment, the seven planes (FIG. 5)perpendicular to the first direction D1 and seven planes (FIG. 8)perpendicular to the second direction D2, i.e. the cut faces in StepsS102, S106, are planes inclined by about 45 degrees with respect to eachside of a pair of opposing faces S1, S2 of the original block 600. Bycutting the first and second small blocks along such planes, as shown inFIG. 11, it is possible to obtain a plurality of cross dichroic prisms500 without wasting the glass that forms the original block 600.

FIG. 12 is an explanatory diagram showing one manufactured crossdichroic prism 500 enlarged. FIG. 12 shows the cross dichroic prism 500seen from the same direction as in FIG. 2. The cross dichroic prism 500comprises four rectangular prisms 501-504. Each rectangular prism is anangular column prism having a bottom face with a substantially rightangle isosceles triangular shape. At the almost “X” shaped interfaceformed by the four rectangular prisms 501-504, red light reflecting filmRR and blue light reflecting film RB are formed. Specifically, bluelight reflecting film RB is formed on the third and fourth rectangularprisms 503, 504, attached to the first and second rectangular prisms501, 502 via an adhesive layer ALB. Continuous red light reflecting filmRR is formed on the first pair of rectangular prisms 501, 504, attachedto the second pair of rectangular prisms 502, 503 via the adhesive layerALR. In FIG. 12, thicknesses of blue light reflecting film RB, red lightreflecting film RR, and adhesive layers ALB, ALR are portrayedconsiderably exaggerated, for convenience in explanation.

As shown in FIG. 12, red light reflecting film RR is formed so as toextend continuously over the two rectangular prisms 501, 504, whereasblue light reflecting film RB is divided over the rectangular prisms503, 504. This configuration is due to the fact that the blue lightreflecting film RB (Step S104) is formed prior to the red lightreflecting film RR (Step S108). In this embodiment, the reason forforming the two kinds of selective film in this order is to improve theoptical characteristics of the cross dichroic prism.

Specifically, if a cross dichroic prism is manufactured by the procedureshown in FIG. 3, the first selective film (blue light reflecting filmRB) formed previously must necessarily be segmented in Step S106. Whensubsequently sticking the second small blocks together in Step S109, thesegmented two first selective films may in some instances not bearranged in the same plane. In the case that such a cross dichroic prismis used in a projector, linear striation due to segmentation of thefirst selective film formed previously may appear in the image projectedonto the screen SC, so that the image is not expressed continuously. Inthis embodiment, to avoid as much as possible the occurrence ofconspicuous linear striation, the blue light reflecting film RB isformed prior to the red light reflecting film RR. That is, thesensitivity of the human eye (visibility) is higher in the order: greenlight, red light, blue light. Accordingly, in this embodiment, bypreviously forming the blue light reflecting film RB, corresponding toblue light, which is not readily conspicuous, segmentation of the firstselective film formed previously does not stand out. By so doing, animage can be expressed continuously.

As will be understood from the preceding explanation, in the case thattwo kinds of selective film, i.e. a green light reflecting film and ared light reflecting film, and in the case that two kinds of selectivefilm, i.e. a green light reflecting film and a blue light reflectingfilm, are formed on a cross dichroic prism 500, it is preferable to setthe green light reflecting film as the second selective film formedlater.

In this embodiment, the first selective film (blue light reflecting filmRB) and the second selective film (red light reflecting film RR) can beformed by laminating a dielectric film using an ion plating method, anion assist method, a sputter method etc. The first selective film isformed in a state in which the first small block is heated to about 200°C. in a chamber, whereas the second selective film is formed in a statein which the second small block is heated to about 100° C. The reasonthat setting temperature differs between the first small blocks andsecond small blocks in this way is that the second small blocks includethe adhesive layers ALB, together with the first selective film (bluelight reflecting film RB). If the adhesive layer is heated, adhesivestrength declines, and light transmittance declines as well. Thus, inthis embodiment, the second small blocks are set to a relatively lowtemperature during the formation of the second selective film.

As described above, in this embodiment, cross dichroic prism 500 isproduced from a single original block 600. Thus, variation in therefractive index of light-transmissive parts constituting the crossdichroic prism that occurs due to differences among lots etc. can bereduced, and reflection of light at the interface of the two rectangularprisms due to a difference of refractive index can be reduced. As aresult, it is possible to improve the optical characteristics of thecross dichroic prism.

Also, as described above, the original block 600 prepared in thisembodiment is produced by press forming of melting glass using a mold.With such an original block 600, it frequently happens that internalstrain is generated during solidification of the melting glass,resulting in spatial variations in refractive index. However,differences in refractive index are relatively small in portions thatare spatially proximal to one another. Thus, in Step S105 of thisembodiment, the eight first small blocks are stuck together in such away that each portion of the light-transmissive member constituting eachfirst small block 611-618 is placed in the same location within theoriginal block 600. The same is true as regards the second small blocks621-628 in Step S109. By so doing, the refractive index of the fourrectangular prisms constituting each cross dichroic prism 500 obtainedin Step S110 can be substantially uniform. Thus, reflection of light atthe interface of the two rectangular prisms due to a difference ofrefractive index can be further reduced. As a result, it is possible tofurther improve the optical characteristics of the cross dichroic prism.

Further, since the cross dichroic prism is manufactured withoutpreparing four rectangular prisms separately as in the prior art, thereare the following advantages.

When four rectangular prisms are prepared separately as in the priorart, there are instances in which “roundness” and “chipping” occur inthe apex angle portion of rectangular prisms situated in the center ofthe cross dichroic prism. This is because it is difficult toindividually cut the apex angle portion of each rectangular prism toabout 90 degrees. Such “roundness” and “chipping” in the apex angleportion causes scattering of modulated light passing through thevicinity of the center of the cross dichroic prism, and generates linearstriation and spot-like shadow in images. With the method of thisembodiment, on the other hand, the formation to about 90 degrees of theapex angle portion of each rectangular prism 501-504 is possible withoutindividually cutting the apex angle portion of each rectangular prism501-504, so “roundness” and “chipping” of the apex angle portion can bereduced. That is, with this embodiment, by forming the cross dichroicprism without individually cutting apex angle portions, the opticalcharacteristics of the cross dichroic prism 500 can be improved, and asa result, images can be expressed continuously.

The first selective film (blue light reflecting film RB) included in thecross dichroic prism 500 is segmented into two, and the two segmentedfirst selective films are formed simultaneously on one first smallblock. Therefore, the two segmented first selective films havesubstantially the same optical characteristics. When four rectangularprisms are prepared separately as in the prior art, there are instancesin which the optical characteristics (reflection characteristics andtransmission characteristics) of the two first selective films includedin the cross dichroic prism are different. In such instances, color inan image displayed on screen SC may differ on both sides of acenterline. That is, in this embodiment, two kinds of selective film RB,RR included in a single cross dichroic prism are each formedsimultaneously, improving the optical characteristics of cross dichroicprism 500, as a result of which it is possible to make colordistribution within an image substantially uniform.

Similarly, the adhesive layer ALB adjacent to the first selective film(blue light reflecting film RB) included in cross dichroic prism 500 issegmented into two as well as the first selective film, and the twosegmented adhesive layers ALB are formed simultaneously at the interfaceof two adjacent first small blocks. Therefore, the two segmentedadhesive layers ALB are formed with substantially the same thickness.When four rectangular prisms are prepared separately as in the priorart, there are instances in which adhesive layers situated adjacent totwo first selective films included in cross dichroic prism differ inthickness. In such instances it becomes difficult to arrange the twoselective films in the same plane. In such cases, as described above,linear striation appears in an image displayed on screen SC so that theimage is not expressed continuously. That is, with the presentembodiment, adhesive layers ALB, ALR adjacent to the two kinds ofselective film RB, RR included in a single cross dichroic prism 500 areeach formed simultaneously, whereby the optical characteristics of crossdichroic prism are improved, as a result of which it is possible todisplay an image continuously.

B. Second Embodiment

In the first embodiment, as shown in FIG. 2, the second polarizingplates 302Ro, 302Go, 302Bo at the light exiting sides of the liquidcrystal light valves 300R, 300G, 300B are attached to light transmissivesubstrates 308, but may instead be attached to the cross dichroic prism500. By so doing, the three light transmissive substrates 308 can beomitted.

In the way, when modulated light emitted from liquid crystal panels301R, 301G, 301B enters the polarizing plates 302Ro, 302Go, 302Bo, theyblock light components other than a predetermined polarized component,and therefore generate heat. Such heat generation can lead todeterioration of polarizing plates, and it is therefore desirable forpolarizing plate temperature to be as low as possible.

Thus, when polarizing plates 302Ro, 302Go, 302Bo are attached to a crossdichroic prism, it is preferable that the cross dichroic prism is madeof a light transmissive member having relatively high thermalconductivity. Accordingly, in this embodiment, cross dichroic prisms aremanufactured using a monocrystalline sapphire member of relatively highthermal conductivity as a light transmissive member.

FIG. 13 is an explanatory diagram showing a cross dichroic prism 510manufactured using a monocrystalline sapphire member. As shown in thedrawing, on the three light incident faces of this cross dichroic prism510 there are attached second polarizing plates 302Ro, 302Go, 302Boconstituting the liquid crystal light valves 300R, 300G, 300B.

In FIG. 13, linearly polarized lights emitted from two polarizing plates302Ro, 302Bo are s-polarized light whose electric vector oscillatesparallel to the y axis in the drawing, and linearly polarized lightemitted from the other one polarizing plate 302Go is p-polarized lightwhose electric vector oscillates parallel to the x axis in the drawing.The efficiency of utilization of light in the cross dichroic prism 510can be increased by causing linearly polarized light to enter. That is,the reflection characteristics of the two reflecting films RB, RR formedin cross dichroic prism 510 are better with s-polarized light than withp-polarized light, while conversely, light transmission characteristicsare better with p-polarized light than with s-polarized light. Thus,s-polarized light is designated as the light to be reflected by the tworeflecting films RB, RR, and p-polarized light is designated as thelight to be transmitted through the two reflecting films RB, RR.

Incidentally, monocrystalline sapphire is a uniaxial crystal whose axis,called the c axis, is the optic axis. With monocrystalline sapphire, therefractive index in the c axis direction and the refractive index in thedirection perpendicular to the c axis are different. The cross dichroicprism 510 shown in FIG. 13 is manufactured in such a way that the c axisof the monocrystalline sapphire is substantially aligned with thedirection of the intersection line (direction y in the drawing) of thetwo kinds of selective film RB, RR formed at the interface ofrectangular prisms 511-514. The c axis of the monocrystalline sapphireis set in this way so that the polarization state of linearly polarizedlight (s-polarized light or p-polarized light) entering the crossdichroic prism 510 is unchanged.

Specifically, when linearly polarized light enters a uniaxial crystal,in some instances it is changed to elliptically polarized light due tobirefringence. However, as shown in FIG. 13, where the travelingdirection of linearly polarized light is substantially perpendicular tothe optical axis (c axis), and the electric vector of the linearlypolarized light is set to be substantially parallel or perpendicular tothe optic axis (c axis), the linearly polarized light is emitted withits polarization state substantially unchanged.

FIG. 14 is an explanatory diagram showing a block 610 formed of amonocrystalline sapphire member prepared to obtain the cross dichroicprism 510 of FIG. 13. As shown in the drawing, the block 610 of thisembodiment also has an outer shape of a substantially rectangularparallelopiped form, and the c axis of the monocrystalline sapphire isset so as to be parallel to one side of block 610.

The cross dichroic prism 510 can then be manufactured by a proceduresimilar to the first embodiment (FIG. 3). In FIG. 14, the faces to becut in Steps S102, S106, S110 of FIG. 3 are shown by dashed lines. Thefirst direction D1 and second direction D2 that prescribe the cutting inStep S102, S106 are set to a substantially perpendicular direction withrespect to the c axis of the monocrystalline sapphire.

Instead of using a monocrystalline sapphire member, a rock crystalmember can be used. Here, rock crystal refers to monocrystalline SiO₂.Rock crystal, like monocrystalline sapphire, is a uniaxial crystal.Thus, the cross dichroic prism is preferably manufactured, as in FIG.13, in such a way that the optic axis of the rock crystal (called the zaxis) is substantially aligned with the direction of the intersectionline (direction y in the drawing) of the two kinds of selective filmformed at the almost X shaped interface of the rectangular prisms.

In this embodiment, the case of manufacturing cross dichroic prisms byusing a monocrystalline sapphire member or rock crystal member, whichare uniaxial crystals has been described, but other light transmissivemembers of relatively high thermal conductivity could be used as well.By using such light transmissive members, temperature rise of the crossdichroic prism per se can be reduced, and temperature rise of polarizingplates due to heat generation of polarizing plates attached to the crossdichroic prism can be reduced significantly. Generally, as lighttransmissive members of relatively high thermal conductivity, it ispreferable to use members having thermal conductivity of at least about5.0 W/(m·K), such as the above-mentioned monocrystalline sapphire memberor rock crystal member.

C. Third Embodiment

When manufacturing a cross dichroic prism in the manner described in thefirst embodiment, the first selective film (blue light reflecting filmRB) formed previously must necessarily be segmented in Step S106. Inthis case, when sticking the second small blocks together in Step S109,the segmented two first selective films may in some instances not bearranged in the same plane.

FIG. 15 is an explanatory diagram showing a first example of a crossdichroic prism in which the segmented two first selective films are notarranged in the same plane. In this cross dichroic prism 500Z1, thesegmented two first selective films RBa, RBb are formed substantiallyperpendicular to the polished cut face of the first pair of rectangularprisms 501, 504 and the polished cut face of the second pair ofrectangular prisms 502, 503, respectively. The two first selective filmsRBa, RBb are substantially parallel to each other, but are not arrangedwithin the same plane.

FIG. 16 is an explanatory diagram showing a second example of a crossdichroic prism in which the segmented two first selective films are notarranged in the same plane. In this cross dichroic prism 500Z2, thesegmented two first selective films RBa, RBb are formed somewhatinclined with respect to the polished cut face of the first pair ofrectangular prisms 501, 504 and the polished cut face of the second pairof rectangular prisms 502, 503, respectively. The two first selectivefilms RBa, RBb are arranged so as to intersect each other, and are notarranged within the same plane.

When such a cross dichroic prism 500Z1, 500Z2 is used in a projector, insome instances an image projected onto a screen SC can not be expressedcontinuously. Accordingly, in this embodiment, the manufacturing methodis manipulated so that the segmented two first selective films areplaced within the same plane. That is, the manufacturing method of thisembodiment is similar to that of the first embodiment (FIG. 3), but theouter shapes of the first and second processed blocks obtained in StepsS105, S109 are modified.

FIG. 17 is an explanatory diagram showing a first processed block 602Aobtained in Step S105 of the third embodiment, and corresponds to FIG.7. As shown in the drawing, the plurality of first small blocks 611-618are stuck together with adjacent two first small blocks alternatelydislocated by a predetermined dimension in a third direction D3 in thedrawing. Here, the third direction is a direction substantiallyperpendicular to the first and second directions D1, D2 (FIG. 5, FIG.8). The first small blocks 611-681 are stuck together such that eachportion of the light transmissive member constituting each small blockis placed in substantially the same location within the original block600.

In Step S106, as in the first embodiment, by cutting the first processedblock 602A, a plurality of second small blocks are obtained. FIG. 18 isan explanatory diagram showing the aspect of cutting the first processedblock 602A, and corresponds to FIG. 8. As shown in the drawing, thefirst processed block 602A is cut along seven planes indicated by dashedlines, so as to obtain eight second small blocks 631-638.

In Step S107, the cut faces of the second small blocks 631-638 arepolished. In the way, in this embodiment the plurality of first smallblocks 611-618 are stuck together with adjacent two first small blocksalternately dislocated. Thus, this “dislocation” can be utilized whenperforming the polishing process. FIG. 19 is an explanatory diagramdepicting a representative aspect of the polishing process. As shown inthe drawing, polishing device 800 comprises a polishing stage 810 and aholder 820. The bottom face 820S of the holder is set parallel to thetop face 810S of the polishing stage. During the polishing process, thepolishing stage rotates about a shaft that is not shown, and the holderdescends towards the polishing stage. In FIG. 19, the No. 4 second smallblock 634 shown in FIG. 18 is the processing target. The holder 820holds the second block 634 by using polishing jigs 830 of columnar shapehaving a right triangular bottom face. One of the two rectangular facesof polishing jig 830 contacts the dislocated face of small block 634,and the other contacts the bottom face 820S of the holder. The holder820, polishing jig 830 and small block 634 are joined by means ofadhesive. In this way, by using the dislocated face as a reference facefor the polishing process, the cut face of small block 634 can bepolished in a direction perpendicular with respect to the dislocatedface (i.e., the face on which the first selective film is formed). Bymeans of this, the problem described in FIG. 16 can be avoided, and thesegmented two first selective films RBa, RBb can be formed substantiallyperpendicular to the polished cut faces of the pair of rectangularprisms.

Later, in Step S108, second selective films (red light reflecting filmsRR) are formed for the eight second small blocks 631-638. Then, in StepS109, the eight second small blocks 631-638 are stuck together withadhesive so as to produce a second processed block.

FIG. 20 is an explanatory diagram showing a second processed block 602Bobtained in Step S109 of the third embodiment, and corresponds to FIG.10. As shown in the drawing, the plurality of second small blocks631-638 are stuck together so as to reproduce the outer shape of thefirst processed block 602A shown in FIG. 17. The second small blocks631-638 are stuck together so that each portion of the lighttransmissive member constituting each small block is placed in the samelocation within the first processed block 602A. In the way, in thisembodiment, the aforementioned “dislocation” can be used when performingthe sticking together process of the plurality of second small blocks.That is, the second small blocks 631-638 can be stuck together in astate such that the dislocated faces included in the second small blocks631-638 are aligned in the same plane by using a jig. By this means, theproblems described in FIGS. 15 and 16 can be avoided, and the twoselective films RBa, RBb segmented due to cutting out of the pluralityof second small blocks can be easily arranged within the same plane.

Subsequently, in Step S110, by cutting the second processed block 600B,a plurality of cross dichroic prisms are obtained.

As described above, in this embodiment, in Step S105 (FIG. 17), theplurality of first small blocks 611-618 are stuck together in a state inwhich adjacent two first small blocks are alternately dislocated in thethird direction. Also, in Step S109 (FIG. 20), the plurality of secondsmall blocks 631-638 are stuck together so as to reproduce the outershape of the first processed block 602A. By so doing, as shown in FIG.12, it is possible to obtain relatively easily a cross dichroic prism inwhich the segmented two first selective films are arranged in the sameplane.

In this embodiment, the plurality of first small blocks 611-618 arestuck together in a state in which adjacent two first small blocks arealternately dislocated in the third direction D3, but instead can bestuck together sequentially dislocated in the third direction D3.However, by employing this embodiment, there is the advantage that theamount of wasted light transmissive member can be reduced when obtainingthe plurality of cross dichroic prisms in Step S110.

As described in the second embodiment, a cross dichroic prism may bemanufactured by using uniaxial crystal members such as a monocrystallinesapphire member or a rock crystal member. In this case, it is preferablethat the first direction D1 and second direction D2 that prescribecutting in Step S102, S106 are set to a substantially perpendiculardirection with respect to the optic axis of the monocrystalline sapphireor rock crystal (c axis or z axis). That is, the optic axis should beset parallel to the third direction D3 in FIG. 17.

The present invention is not restricted to the above embodiments or itsmodifications, but there may be many other modifications, changes, andalterations without departing from the scope or spirit of the maincharacteristics of the present invention. Some examples of possiblemodification are given below.

(1) In the second embodiment, the second polarizing plates on the lightexiting sides of the liquid crystal light valves are attached to thethree light incident faces of the cross dichroic prism. In such a case,it is preferable that, of the six face of the cross dichroic prism, atleast one of the two faces through which light does not pass (i.e. thebottom face and top face) be placed in contact with a heat sink. Forexample, a metal cooling fin could be joined to the top face, and ametal base frame that carries the projector 1000 joined to the bottomface. By so doing, temperature rise due to heat generation of thepolarizing plates can be further reduced.

In the above embodiments, a polarizing plate is provided on the lightexiting side of each liquid crystal light valve, but in some cases aretardation plate is provided. In such a case, the retardation plate maybe attached to the light incident face of the cross dichroic prism. Byso doing, temperature rise due to heat generation of the retardationplate can be reduced.

(2) In the above embodiments, each first selective film (blue lightreflecting film RB) is formed on one cut face of each of the seven firstsmall blocks 612-618, but could instead be formed, for example, on bothcut faces of three, i.e. No. 2, No. 4 and No. 6, first small blocks 612,614, 616, and on one cut face of No. 8. The same is true of the secondselective films (red light reflecting films RR).

Generally, the first selective film will be formed on the cut faces ofat least some of the plurality of first small blocks. Specifically, afirst selective film will be formed such that the first selective filmis placed at the interface of adjacent two first small blocks within afirst processed block obtained by sticking together a plurality of firstsmall blocks. The same is true of the second selective film.

(3) In the above embodiments, in Step S102 of FIG. 3, the block is cutalong a plurality of planes perpendicular to a first direction so as toobtain a plurality of first small blocks, but instead of this, the blockmay be cut along a single plane perpendicular to a first direction so asto obtain two first small blocks.

In Step S106, the first processed block is cut along a plurality ofplanes perpendicular to a second direction so as to obtain a pluralityof second small blocks, but instead of this, the first processed blockmay be cut along a single plane perpendicular to a second direction soas to obtain two second small blocks.

In this way, by cutting along a single plane in Steps S102, S106, only asingle almost X shaped interface appears in the second processed block.In this case, at least one cross dichroic prisms can be obtained fromthe second processed block. That is, in the case that the secondprocessed block is relatively short in the direction of the intersectionline of the two kinds of selective film formed at the almost X shapedinterface, a single cross dichroic prism may be obtained, and whenrelatively long, at least two cross dichroic prisms can be obtained.

Generally, a plurality of first small blocks will be obtained by cuttingan original block along at least one plane perpendicular to a firstdirection. Also, a plurality of second small blocks will be obtained bycutting a first processed block along at least one plane perpendicularto a second direction. That is, the invention permits forming at leastone light-selective prism by means of cutting a single block in theabove-described manner.

(4) In the above embodiments, cross dichroic prism 500 is used as acolored light combining optical system to combine three colored lights,but by reversing the traveling direction of light, it could be used alsoas a colored light separating optical system. That is, by causing whitelight to enter the light exiting face of cross dichroic prism 500 andcausing three colored lights to exit from the light incident faces ofcross dichroic prism 500, utilization as a colored light separatingoptical system is possible. Accordingly, it is possible to use thisprism in place of the colored light separating optical system 200 inFIG. 1.

Generally, the present invention is applicable to the manufacture of alight-selective prism, which has a substantially regular tetragonalcolumnar outer shape and includes two kinds of selective films formed onan almost X shape interface, each selective film selecting colored lighthaving wavelengths of a predetermined range.

Industrial Applicability

This invention is applicable to the manufacture of a projector forprojecting and displaying images.

What is claimed is:
 1. A method for manufacturing a light-selectiveprism having a substantially regular tetragonal columnar outer shape andincluding two kinds of selective films formed on an almost X shapeinterface of the prism, each selective film selecting colored lighthaving wavelengths of a predetermined range, the manufacturing methodcomprising the steps of: (a) preparing a block formed of a lighttransmissive member; (b) cutting the block along at least one planeperpendicular to a first direction so as to obtain a plurality of firstsmall blocks whose dimension in the first direction is substantiallyequal to a predetermined dimension; (c) forming a first selective filmon a cut face of at least one of the plurality of the first smallblocks; (d) sticking the plurality of the first small blocks together soas to obtain a first processed block in which the first selective filmis situated at an interface of adjacent two first small blocks; (e)cutting the first processed block along at least one plane perpendicularto a second direction that is substantially perpendicular to the firstdirection so as to obtain a plurality of second small blocks whosedimension in the second direction is substantially equal to thepredetermined dimension; (f) forming a second selective film on a cutface of at least one of the plurality of the second small blocks; (g)sticking the plurality of the second small blocks together so as toobtain a second processed block in which the second selective film issituated at an interface of adjacent two second small blocks; and (h)obtaining at least one light-selective prism from the second processedblock.
 2. The manufacturing method according to claim 1, wherein thestep (b) includes polishing cut faces of the first small blocks; and thestep (c) includes polishing cut faces of the second small blocks.
 3. Themanufacturing method according to claim 1, wherein the step (h) includesculling the second processed block so as to obtain a plurality of thelight-selective prisms.
 4. The manufacturing method according to claim1, wherein the step (d) includes sticking the plurality of the firstsmall blocks together such that the outer shape of the block isreproduced; and the step (g) includes sticking the plurality of thesecond small blocks together such that the outer shape of the black isreproduced.
 5. The manufacturing method according to claim 4, whereinthe step (d) includes sticking the plurality of the first small blockstogether such that each portion of the light transmissive memberconstituting each first small block is placed in the same locationwithin the block; and the step (g) includes sticking the plurality ofthe second small blocks together such that each portion of the lighttransmissive member constituting each second small block is placed inthe same location within the block.
 6. The manufacturing methodaccording to claim 1, wherein the step (d) includes sticking theplurality of the first small blocks together in a state in whichadjacent two first small blocks are dislocated in a directionsubstantially perpendicular to the first and tho second directions; andthe step (g) includes sticking the plurality of the second small blockstogether such that an outer shape of the first processed block isreproduced.
 7. The manufacturing method according to claim 6, whereinthe step (d) includes sticking the plurality of the first small blockstogether such that each portion of the light transmissive memberconstituting each first small block is placed in substantially the samelocation within the block; and the step (g) includes sticking theplurality of the second small blocks together such that each portion ofthe light transmissive member constituting each second small block isplaced in the same location within the first processed block.
 8. Themanufacturing method according to claim 1, wherein the first selectivefilm is a blue light reflecting film for selectively reflecting bluelight; and the second selective film is a red light reflecting film forselectively reflecting red light.
 9. The manufacturing method accordingto claim 1, wherein the block has a substantially rectangularparallelopiped shape; and the at least one plane perpendicular to thefirst direction and the at least one plane perpendicular to the seconddirection are set to planes inclined by about 45 degrees with respect toeach side of one pair of opposing faces of the block.
 10. Themanufacturing method according to claim 1, wherein the lighttransmissive member is a member having a thermal conductivity of atleast about 5.0 W/(m·K).
 11. The manufacturing method according to claim10, wherein the light transmissive member is a uniaxial crystal member;and the first and second directions are set to directions substantiallyperpendicular to an optic axis of the uniaxial crystal.
 12. Themanufacturing method according to claim 11, wherein the uniaxial crystalmember is a monocrystalline sapphire member.
 13. The manufacturingmethod according to claim 11, wherein the uniaxial crystal member is arock crystal member.